Usually during postdoc application in my field applicants are asked to arrange three letters of recommendation. Sometimes one can add more than three letters. Recently I visited an institute for two weeks and gave a seminar on my work. There I also interacted with a faculty member who is a very famous. I asked him if he could write a letter for me. He told me that he could write one. But that would be very short since he knew me only for two weeks. He asked me to take letters from people who knew me for few years and also said that he could write a "fourth letter" if I insist.
Will it be a good idea to ask a letter from him?
(From my personal experience and after discussing with some other people who know him very well, I know he is an excellent human being and also very friendly.)